Sputter target/backing plate assembly and method of making same

ABSTRACT

A method of forming a sputter target/backing plate assembly comprises the steps of: providing a target fabricated from a first material having a coefficient of thermal expansion; providing a backing plate fabricated from a second material having a coefficient of thermal expansion; providing a block fabricated from a third material having a coefficient of thermal expansion; positioning the block on one side of the backing plate; positioning the target on the other side of the backing plate; and subjecting the target, backing plate and block to elevated temperature and pressure to bond the target, backing plate and block together. The third material is selected so as to have a coefficient of thermal expansion which counteracts the effects of the coefficients of thermal expansion of the first and second materials. The third material may be selected so as to have a coefficient of thermal expansion which is approximately the same as the coefficient of thermal expansion of the first material.

This application is a divisional of application Ser. No. 08/515,857,filed Aug. 16, 1995, now U.S. Pat. No. 5,857,611.

FIELD OF THE INVENTION

The present invention relates generally to sputtering, and moreparticularly to sputter target/backing plate assemblies utilized insputtering operations.

BACKGROUND OF THE INVENTION

Sputtering is the deposition of a thin film of a desired material on asubstrate. Sputtering is employed to manufacture semi-conductor devicessuch as integrated circuits. In a sputtering operation, the material tobe deposited on the substrate is removed from a sputter target bybombarding the target with ions. In addition to removing material fromthe target for deposit on the substrate, ion bombardment transfersthermal energy to the target. It is therefore generally necessary toprovide a means of cooling the target during the sputtering operation toprevent the target from overheating.

The target is traditionally mounted to a backing plate. Cooling liquidis circulated in heat transfer contact with the backing plate to removeheat transferred to the target by the bombarding ions. It is importantthat there be good thermal, as well as structural, bonding between thetarget and backing plate to ensure adequate heat transfer from target tobacking plate to coolant and to ensure the structural integrity of thetarget/backing plate connection during sputtering.

One technique for bonding the target to the backing plate is known asdiffusion bonding. In diffusion bonding the target/backing plate bond isproduced by pressing the material surfaces into intimate contact whileapplying heat to induce metallurgical joining and diffusion to varyingextent across the bond interface. Bonding aids, metal combinations whichare more readily joined, are sometimes applied to one or both of thesurfaces to be bonded. Such coatings may be applied by electroplating,electroless plating, sputtering, vapor deposition or other usabletechnique for depositing an adherent metallic film. It is also possibleto incorporate a metallic foil between bonding members which foil hasthe ability to be more easily bonded to either of the material to bejoined. The surfaces to be joined are prepared by chemical or othermeans to remove oxides or their chemical films which interfere withbonding.

The target/backing plate assembly may be diffusion bonded usingtechniques such as hot isostatic pressing (“HIPing”) or uniaxial hotpressing (“UHPing”). In UHPing, the unbonded assembly is placed betweena pair of plungers, platens or rams. These rams are contained within acontrol chamber which allows for the control of temperature, pressureand other atmospheric conditions. The controlled atmosphere is a vacuum,reducing gas or inert gas, or a combination thereof. The temperature inthe uniaxial hot press control chamber is increased in order to heat theunbonded assembly. The assembly is heated to a temperature somewhatbelow the homologous melting point of the metal used for the backingplate. By elevating the temperature of the assembly to a temperaturesomewhat below the melting point of the backing plate material, thebacking plate softens, and upon pressing, forms a tight interface withthe treated bonding surface of the sputter target. As the assembly isheated, a compressing force is applied on the assembly by the rams in auniaxial direction. The assembly is maintained in the control chamberunder these temperature, pressure and atmospheric gas conditions for aperiod of time sufficient to form the bonded sputter target/backingplate assembly.

In HIPing, the treated sputter target and backing plate are oriented toform an assembly having an interface defined by the bonding surfaces,and this assembly is placed within a HIPing canister. Once the assemblyis placed in the HIPing canister a vacuum is pulled on the canister andthe canister is then placed within a HIPing chamber. The ambientatmosphere in the HIPing chamber is replaced with a true inert gas, suchas argon or helium. In addition, the temperature and pressure in theHIPing chamber are increased as discussed above with respect to UHPing,in order to form a bonded sputter target/backing plate assembly. Againthe assembly is heated to a temperature somewhat below the homologousmelting point of the metal used for the backing plate, and the HIPingcanister and assembly contained therein are compressed from all sides atelevated pressure. The assembly is maintained at the desiredtemperature, pressure and atmospheric conditions for a sufficient periodto form the bonded target/backing plate assembly.

Representative times, temperatures and pressures for UHPing and HIPingare disclosed in U.S. Pat. No. 5,836,506, issued Nov. 17, 1998, entitled“Sputter Target/Backing Plate Assembly And Method Of Making Same”,assigned to the assignee of the present invention and herebyincorporated by reference herein as fully set forth in its entirety.

Due to coefficient of thermal expansion induced shrinkage of the bondedtarget/backing plate during cooling subsequent to the diffusion bonding,any mismatch between the thermal expansion coefficients of the targetand backing plate creates stresses in the assembly which can distort theassembly and/or crack the target.

One particular material from which targets are fabricated istungsten-titanium. Target/backing plate assemblies wherein the target isfabricated from tungsten-titanium have typically employed titanium asthe backing plate material to avoid a coefficient of thermal expansionmismatch between the target and backing plate. Titanium is expensive,however, and is thus not desirable as a material from which to fabricatebacking plates. Less expensive materials, such as aluminum, havesignificantly different thermal expansion coefficients from that oftungsten-titanium though, resulting in target/backing plate assemblydistortion and/or target cracking.

It is therefore a main objective of the present invention to provide atarget/backing plate assembly and method of making same which canaccommodate mismatch between target and backing plate coefficients ofthermal expansion so as to not jeopardize the thermal and/or structuralbond integrity between the target and backing plate.

SUMMARY OF THE INVENTION

The present invention attains the stated objective by providing a methodof forming a sputter target/backing plate assembly. The method comprisesthe steps of: providing a target fabricated from a first material havinga coefficient of thermal expansion; providing a backing plate fabricatedfrom a second material having a coefficient of thermal expansion;providing a block fabricated from a third material having a coefficientof thermal expansion; positioning the block on one side of the backingplate; positioning the target on the other side of the backing plate;and subjecting the target, backing plate and block to elevatedtemperature and pressure to bond the target, backing plate and blocktogether. The third material is selected so as to have a coefficient ofthermal expansion which counteracts the effects of the coefficients ofthermal expansion of the first and second materials.

The third material may be selected so as to have a coefficient ofthermal expansion which is approximately the same as the coefficient ofthermal expansion of the first material.

The target, backing plate and block may be diffusion bonded together viahot isostatic pressing or uniaxial hot pressing.

The target is preferably fabricated from tungsten-titanium, the backingplate is preferably fabricated from aluminum and the block is preferablyfabricated from graphite. In that event, the target has a coefficient ofthermal expansion of approximately 6.4×10⁻⁶/° C., the backing plate hasa coefficient of thermal expansion of approximately 19.8×10⁻⁶/° C. andthe block has a coefficient of thermal expansion of approximately5.8×10⁻⁶/° C.

The backing plate preferably has a recess machined therein and the blockis disposed in the recess.

The backing plate may have a hub in the center of the recess. In thatevent, the block has a bore therethrough for receiving the backing platehub.

The present invention also provides a target/backing plate assemblyformed by the method.

The major advantage of the present invention is that a target/backingplate assembly and method of making the same is provided which allowstargets and backing plates fabricated from dissimilar materials havingdissimilar thermal expansion coefficients to be diffusion bonded withoutsuffering assembly distortion or target cracking during cooling.

These and other objects and advantages of the present invention willbecome more readily apparent during the following detailed descriptiontaken in conjunction with the drawings herein, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B diagrammatically illustrate the series of stepsperformed in accordance with the method of forming a sputtertarget/backing plate assembly of the present invention;

FIG. 2 illustrates a sputter target/backing plate assembly made inaccordance with the method of FIGS. 1A and 1B;

FIGS. 3A and 3B diagrammatically illustrate the series of stepsperformed in accordance with the method of forming an alternative formof sputter target/backing plate assembly of the present invention; and

FIG. 4 illustrates a sputter target/backing plate assembly made inaccordance with the method of FIGS. 3A and 3B.

DETAILED DESCRIPTION OF THE INVENTION

Referring first to FIG. 1A, there is illustrated an assembly 10 forloading into a HIPing canister for subsequent subjection to HIPing.

The assembly 10 includes an aluminum backing plate 12 which includes arecess 14 machined into one side thereof and a tungsten-titanium target16 for bonding to the backing plate 12 at a bond interface 18. Agraphite block 20 is provided for insertion into recess 14. A dummyaluminum backing plate 22 is positioned above the target 16. Upper andlower steel plates 24 and 26 respectively are positioned above the dummybacking plate 22 and below the backing plate 12 with block 20 therein.

As illustrated in FIG. 1B, the assembly 10, which would be contained ina HIPing canister (not shown) on which is pulled a vacuum of about 10⁻²torr or greater, is diagrammatically illustrated as being subjected toelevated temperature and pressure as would be the case when contained ina HIPing vessel. Alternatively, FIG. 1B illustrates the assembly 10subjected to elevated temperature and pressure as would be the case whensubjected to UHPing, also at about 10⁻² torr.

The assembly 10 is heated to a temperature of from about 300° C., toabout 575° C., and preferably from about 450° C. to about 550° C., andis pressed to a pressure of from about 30 MPa to about 140 MPa, and ismaintained there for from about 30 minutes to about 60 minutes. IfUHPing is used, the pressure is developed between a pair of plungers,platens or rams. If HIPing is used, the HIPing canister and assembly 10therein is subjected to compressive pressure from all sides in theHIPing vessel. By elevating the temperature of the assembly to atemperature somewhat below the melting point of the backing platematerial the backing plate 12 softens, and upon pressing, forms a tightinterface with the bonding surface of the sputter target 16. Similarly,the block 20 bonds with the underside of the backing plate 12.

During cooling of the assembly 10, the graphite block 20 restrains theshrinkage of the aluminum backing plate 12 so that the thermalcontraction of the composite backing plate 12 and block 20 isapproximately the same as that of the tungsten-titanium target 16.Typically the tungsten-titanium target has a coefficient of thermalexpansion of approximately 6.4×10⁻⁶/° C., the aluminum backing plate hasa coefficient of thermal expansion of approximately 19.8×10⁻⁶/° C. andthe graphite block has a coefficient of thermal expansion ofapproximately 5.8×10⁻⁶/° C. Thus the graphite block 20 has about thesame thermal expansion coefficient as that of the tungsten-titaniumtarget 16. Distortion of the target 16 and backing plate 12 and/orcracking of the target 16 is thereby avoided.

Lastly, as shown in FIG. 2, the cooled assembly 10 has had the steelplates 24 and 26 and dummy backing plate 22 machined away. In addition,the block 20 has been machined from the recess 14 to provide access tothe backing plate 12 by the coolant when installed onto a cathodeassembly.

Referring now to FIG. 3A, an alternative assembly 50 is illustrated. Theassembly 50 includes an aluminum backing plate 52 which includes arecess 54 machined into one side thereof and a central hub 56 forattaching the backing plate 52 to a cathode assembly. Atungsten-titanium target 58 is located within an aluminum spacer ring 60for bonding to the backing plate 52. A graphite block 62 is provided forinsertion into recess 54 and includes a bore 64 therethrough foraccepting the hub 56. A dummy aluminum backing plate 66 is positionedabove the target 58. Upper and lower steel plates 68 and 70 respectivelyare positioned above the dummy backing plate 66 and below the backingplate 52 with block 62 therein.

As in the prior embodiment, the assembly 50 is subjected to the samepressures and temperatures for the same duration by way of either HIPingor UHPing (FIG. 3B). During cooling of the assembly 50, the graphiteblock 62 restrains the shrinkage of the aluminum backing plate 52 sothat the thermal contraction of the composite backing plate 52 and block62 is approximately the same as that of the tungsten-titanium target 58.Distortion of the target 58 and backing plate 52 and/or cracking of thetarget 58 is thereby avoided.

Lastly, and also as in the prior embodiment, as shown in FIG. 4, thecooled assembly 50 has had the steel plates 68 and 70 and the dummybacking plate 66 machined away. In addition, the block 62 has beenmachined from the recess 54 to provide access to the backing plate 52 bythe coolant when installed onto a cathode assembly.

Those skilled in the art will readily recognize numerous adaptations andmodifications which can be made to the present invention which willresult in an improved sputter target/backing plate assembly and method,yet all of which will fall within the spirit and scope of the presentinvention as defined in the following claims. For example, many othermaterials other than tungsten-titanium, aluminum and graphite can beused to fabricate the target, backing plate and block respectively. And,other bonding techniques other than diffusion bonding can be employed.Accordingly, the invention is to be limited only by the scope of thefollowing claims and their equivalents.

What is claimed is:
 1. A sputter target assembly comprising; a targetmember having a first surface, a second surface and a first coefficientof thermal expansion; a backing member having a first surface, anunderside surface opposite the first surface of the backing member and asecond coefficient of thermal expansion and the backing first surface ofthe member being diffusion bonded to the first surface of the targetmember; and a block member having a third coefficient of thermalexpansion and the block member being diffusion bonded to the undersidesurface of the backing member; and the third coefficient of thermalexpansion of the block member being closer to the first coefficient ofthermal expansion of the target member than the second coefficient ofthe thermal expansion of the backing member.
 2. The sputter targetassembly of claim 1 wherein the target member comprisestungsten-titanium; the backing member comprises aluminum; and the blockmember comprises graphite.
 3. The sputter target assembly of claim 1wherein the backing member has a recess in the underside surface intowhich the block member is disposed.
 4. The sputter target assembly ofclaim 3 wherein the backing member has a hub in the center of the recessand the block has a bore therethrough for accommodating the hub of thebacking member.
 5. The sputter target assembly of claim 4 wherein thetarget member comprises tungsten-titanium; the backing member comprisesaluminum; and the block member comprises graphite.
 6. The sputter targetassembly of claim 3 wherein the target member comprisestungsten-titanium; the backing member comprises aluminum; and the blockmember comprises graphite.
 7. The sputter target assembly of claim 1wherein the first coefficient of thermal expansion of the target memberis approximately 6.4×10⁻⁶/° C.; the second coefficient of the thermalexpansion of the backing member is approximately 19.8×⁻⁶/° C.; and thethird coefficient of thermal expansion of the block member isapproximately 5.8×10⁻⁶/° C.
 8. The sputter target assembly of claim 7wherein the target member comprises tungsten-titanium; the backingmember comprises aluminum; and the block member comprises graphite. 9.The sputter target assembly of claim 8 wherein the backing member has arecess in the underside surface into which the block member is disposed.